Like Mikef1182 said, they want the contract and the new service, that's where their money is. If they have the option of selling one outright or with a contract, they'll choose the contract/new subscriber every time.
Oh and the reps will lie to you in the store. I went into the AT&T store to buy my Bold on November 2nd, a couple of days before the launch, and as I was standing there buying my Bold, while it was sitting in the box right on the counter, I heard a rep telling person after person that they didn't even have the Bold in stock. I knew that they had about 11 of them, but they were lying to customers. Maybe they were holding out for their friends. I felt bad for the others and I don't know why the chose to tell me the truth.
BlackBerry Bold 9700